Here's a neat little trick with a killa soundtrack.
Although there was no explanation with the video, I think I can hazard a guess: when the plastic card is placed between the two glasses, the card is moved to the edge, allowing a very small gap. The gap is small enough to allow the liquid through, but the surface tension of the liquid prevents the liquid from spilling. As the sliver of water and whiskey mingle, the whiskey rises to the top, because alcohol has a much lower density than water. After 10 minutes, the two liquids have switched glasses. If you could turn the glasses upside down again without disturbing them, the water and whiskey will change back.